Method and device for producing globular grains of high-puroty silicon having a diameter of between 50 μm and 300 μm and use of the same

ABSTRACT

The invention relates to a method and a device for producing globular grains of high-purity silicon by atomising a silicon melt ( 6 ) in an ultrasonic field ( 10 ). Globular grains having a grain size of 50 μm can be produced by means of said method and device and can be used to separate high-purity silicon from silane in the fluid bed. The silicon melt ( 6 ) is fed into the ultrasonic field ( 10 ) at a distance of &lt;50 mm in relation to a field node, and the atomised silicon leaves the ultrasonic field ( 10 ) at a temperature close to the liquidus point. The invention also relates to a use of the product produced according to the inventive method or using the inventive device, as particles for producing high-purity silicon from silane in a fluid bed.

The present invention relates to a method and a device for producingglobular grains of high-purity silicon having a diameter of between 50μm and 300 μm as well as to the use of the same as seed to separatehigh-purity silicon from silane in a fluid bed.

Thermal degradation of silane (SiH₄) on high-purity silicon seed grainsin a fluid bed has proven as a particularly favourable method forproducing high-purity silicon. In this process, the separated silicongrows on the seed grains thus enlarging the latter. This process may beperformed on a per-arrangement or continuous basis wherein the number ofdischarged final product particles has to be compensated for by feedingnew high-purity silicon seed particles. In most cases, the desired finalproduct of the silane degradation process consists of high-puritysilicon grains having a grain size of between 0.3 mm and 3 mm, as thissize range is particularly suitable for dosing and melting. As aconsequence, the seed particles used should be of a size less than 0.3mm, preferably up to one order of magnitude less than the diameter ofthe desired final product, on the one hand in order to achieve a finalproduct: seed particles ratio as favourable as possible (i.e. as largeas possible), and on the other hand in order to keep the fine-graindischarge from the fluid bed small. Thus, a size range of the seedparticles between 50 μm and 300 μm should be aimed at.

Such seed particles can be obtained, for example, by milling,particularly by breaking or crushing, of high-purity silicon of anyshape and origin to achieve the size required. Patent documents describevarious methods for mechanically milling high-purity silicon:

Methods for milling high-purity silicon in jet mills are known in theart (DE 42 40 749 A1, U.S. Pat. No. 4,424,199). These methods ensure ahigh purity level of the final product. However, the milling result isvery unspecific so that a large percentage of particle material beingsmaller than the desired lower limit of 50 μm is produced. This materialcannot be further utilised and thus is lost.

Further, the milling of high-purity silicon in a cylinder mill is knownin the art. Among the mechanical milling devices, the cylinder mill isbest suited to perform the task. The product's particle size range canbe adjusted very precisely to match the targeted size. However, thereexists the problem of pollution of the final product caused by metalparticles abraded off the mill surface. Expensive purificationtechniques are required (DE 195 29 518 A1, JP 6-144 822-A) to remove theabraded metal particles from the silicon. As an alternative, JP 58-145611-A describes a mill using a silicon roller. The drawback of thisconstruction is that the brittleness of the silicon roller impairs thetechnical availability of the facility.

Patent documents also describe various other methods for producing seedparticles: bombardment of a high-purity silicon target with siliconparticles (U.S. Pat. No. 4,691,866), microwave comminution ofhigh-purity silicon bars (U.S. Pat. No. 4,565,913), electromechanicalcomminution of silicon bars (DE 195 41 539 A1), comminution by means ofelectric discharging (WO 98/07520, DE 195 34 232 A1). A drawback of allthese methods is that they comminute very unspecifically resulting inconsiderable quantities of useless fine particles.

Apart from milling high-purity silicon, the atomisation of a high-puritysilicon melt is also a feasible way to obtain small high-purity siliconparticles. However, the usual method of atomising a high-purity siliconmelt which is drained off a nozzle by lateral gas jetting isuneconomical because very long cooling lines are required. These requirevery large dimensions of the equipment which on the one hand cause highinvestment cost while their adaptation to the relevant purityrequirements is very expensive.

It is therefore an object of the present invention to provide a methodand a device for producing seed particles of appropriate size allowing,without the risk of intrusion of undesired impurities and without undueloss of substance, the production of seed particles consisting ofhigh-purity silicon of any origin and size distribution.

To this end, a method is suggested wherein the melt is fed in theultrasonic field above a field node, with the atomised silicon leavingthe ultrasonic field at a temperature near the liquefaction point sothat the solidified globular grains essentially adopt a grain size ofbetween 50 μm and 300 μm.

According to the present invention, it is possible to produce suchhigh-purity silicon particles by melting and atomising in an ultrasonicfield without contact with foreign matter implying strong mechanicalforces and without a remarkable percentage of particles of undesired anduseless sizes.

The atomisation of a jet of a liquid substance, preferably a moltenmetal wherein said jet penetrates an ultrasonic field, in particularsaid jet being led through a compressed gaseous medium in the ultrasonicfield, is known in the art (DE 37 35 787 C2). However, this known methodproduces globular grains in a size range of 0.1 μm and less. Further,the state-of-the-art devices fail to meet the purity level required forthe high-purity silicon particles.

The method according to the present invention using the metalloidsilicon allows the production of particles having a size of between 50μm and 300 μm. No relevant quantities of fine particles smaller than 50μm occur. The particles are produced in globular shape at the desiredpurity level.

Preferably, high-purity gases such as hydrogen, rare gas (argon), ornitrogen, are used as inert gas. The gas pressure in the reactionchamber preferably amounts to at least 2 to 40 bar, most preferably 10bar.

Finally, the scope of the present invention covers a utilisation of theproduct made in accordance with the method according to the presentinvention or using the device according to the present invention as seedparticles for producing high-purity silicon from silane in a fluid bed,as the high-purity silicon particles delivered from the pressurisedultrasonic atomisation device can be used as such for this purposewithout any subsequent treatment. These particles are characterised byglobular shapes, a crystalline structure and high purity and thus aremore suitable than known particles as seed particles in a high-puritysilicon separation process. It is of course possible to further classifyor limit otherwise the grain size before their utilisation.

Hereinafter the present invention will be described in more detail withreference to a drawing representing a preferred example embodiment. Theonly figure schematically shows the structure of a device according tothe present invention.

Silicon 1 is fed by a suitable feeding device, in the example a solidmatter valve 2, continuously or intermittently into a melting pot 4arranged in a pressure vessel 3. Said solid matter valve 2 and thefeeding lines (not shown in detail) are designed so that nocontamination of the silicon 1 can occur. In said melting pot 4, whichconsists preferably of graphite or silicon carbide (SiC) or is providedwith a quartz glass liner, the silicon 1 is melted by means of a heatingmeans 5. The pressure above the melting surface is at least equal to thepressure inside the pressure vessel 3 and ranges preferably between 2and 40 bar.

The pressure vessel 3 is filled with high-purity gas, preferablyhydrogen, rare gas (argon) or nitrogen in order to prevent acontamination of the silicon. The now molten silicon 6 flows through acapillary tube 7 out of said melting pot 4. Said capillary tube 7consists preferably of graphite or SiC and is kept at a temperatureabove the melting temperature of the silicon by a heating means 8. Theheating spiral of said heating means 8 is provided with a heat shield 9.Said heat shield 9 consists of a material not causing any impurities inthe silicon, preferably of graphite or SiC. Said heating means 8protects said capillary tube 7 from freezing, said heat shield 9 reducesthe cooling by the ultrasonic field 10 and prevents any contact betweenmelt drops and heating spiral.

The melting stream flowing out of said capillary tube 7 flows directlyinto a field node of said ultrasonic field 10. At this point, said melt6 is atomised. Said ultrasonic field 10 is generated by preferably twosonotrodes 11 being arranged diametrically opposite to each other whensaid pressure vessel 3 is in use. The sonic frequency ranges between 5and 30 kHz, preferably 20 kHz. Said sonotrodes 11 areresonance-operated. The distance of said sonotrodes 11 should be chosenso wide that the melt drops cannot contact said sonotrodes 11 in theirliquid state; the distance should preferably exceed nine field nodes. Inorder to avoid contamination by the sonotrodes 11, it is possible tocoat them, preferably with SiC. The atomised silicon cools down veryquickly in the ultrasonic field 10. The dimensions of said pressurevessel 3 are chosen so that the atomised silicon does not reach thewalls of said vessel until the atomised silicon has reached its solidstate.

In order to avoid contamination of the silicon due to contact with thejacket of said pressurised vessel 3, the walls are shielded by linersmade of graphite or quartz glass or an appropriate coating such as SiCor equivalent linings. The bottom of said pressure vessel 3 is designedso that the silicon particles enter a discharge channel 12. The siliconparticles are continuously or intermittently discharged from saidpressure vessel 3 via discharge means 13, a suitable valve or a cellularwheel sluice.

The device according to the present invention differs from theconstruction disclosed in DE 37 35 787 C2 in that the discharge openingfor the melt stream (tip of said capillary tube 7) may be positionedvery close, preferably <50 mm, to the central field node of saidultrasonic field 10. As mentioned, this requires the additional heatingdevice 8 of said capillary tube 7 and the heat shield 9 made ofgraphite. Lateral gas jetting towards the melt stream is abandoned. Theatomised silicon leaves the ultrasonic field 10 at a temperature nearthe liquefaction point. Said sonotrodes 11 must be mounted at a verylong distance from each other, preferably longer than nine field nodes.In order to avoid contamination of the silicon, said sonotrodes 11 maybe coated with SiC. The atomised silicon is significantly coarser thanthe material produced according to the state of the art.

1. A device for producing globular grains of high-purity silicon byatomizing a silicon melt in an ultrasonic field inside a pressurizedvessel filled with inert gas, said device comprising: said vesselcontaining a heated melting pot for melting said silicon and a set ofsonotrodes for producing a sonic field, wherein the melt is fed intosaid ultrasonic field through a capillary tube, that the heated portionsaid capillary tube is heat-insulated from said ultrasonic field by aheat shield, and all surfaces which might come into contact with theglobular grains in said pressurized vessel comprise at least one of anon-contaminating material, a coating with a liner made of anon-contaminating material and a coating with a non-contaminatingmaterial.
 2. A device according to claim 1, wherein said ultrasonicfield is generated by at least two sonotrodes being arranged at aminimum distance of nine field nodes between each other.
 3. A deviceaccording to claim 1, wherein said capillary tube has a maximum distanceof 50 mm from one field node of said ultrasonic field.
 4. A deviceaccording to claim 1, wherein said capillary tube is provided with anadditional heating means.
 5. A device according to claim 1, wherein thedistance from a set of walls of said pressurized vessel to anatomisation zone for atomizing said silicon melt is chosen so that theglobular grains are completely solidified before the globular grains hitsaid walls.
 6. A device according to claim 1, wherein at least one ofgraphite, silicon carbide and quartz glass are used as saidnon-contaminating material.
 7. A device according to claim 1, whereinthe silicon is fed into said heated melting pot via a solid mattervalve.
 8. A device according to claim 1, wherein the bottom of saidpressurized vessel is provided with a discharge means for dischargingthe high-purity silicon.
 9. A device according to claim 8, wherein acellular wheel sluice is provided as said discharge means.
 10. A devicefor producing globular grains of high-purity silicon by atomizing asilicon melt in an ultrasonic field inside a pressurized vessel filledwith inert gas, comprising: said vessel containing a heated melting potfor melting said silicon and at least two sonotrodes for producing theultrasonic field; a capillary tube connected to said heated melting potfor feeding the melt into said ultrasonic field, wherein the ultrasonicfield is generated by said sonotrodes being arranged at a minimumdistance of nine field nodes between each other; a heat shield adjacentsaid capillary tube, said heat shield insulating said capillary tubefrom the ultrasonic field; and all surfaces in said pressurized vesselcoming in contact with the globular grains comprise at least one of anon-contaminating material, a coating with a liner made of anon-contaminating material and a coating with a non-contaminatingmaterial.
 11. A device according to claim 10, wherein said capillarytube has a maximum distance of 50 mm from one field node of saidultrasonic field.
 12. A device according to claim 10, wherein saidcapillary tube is provided with a heating means.
 13. A device accordingto claim 10, wherein the distance from the walls of said pressurizedvessel to an atomization zone for atomizing said silicon melt is chosenso that the globular grains are completely solidified before theglobular grains hit said walls.
 14. A device according to claim 10,wherein at least one of graphite, silicon carbide and quartz glass areused as said non-contaminating material.
 15. A device according to claim10, wherein the silicon is fed into said heated melting pot via a solidmatter valve.
 16. A device according to claim 10, wherein the bottom ofsaid pressurized vessel is provided with a discharge means fordischarging the high-purity silicon.
 17. A device according to claim 16,wherein a cellular wheel sluice is provided as said discharge means. 18.A device for producing globular grains of high-purity silicon byatomizing a silicon melt in a sonic field inside a pressurized vesselfilled with inert gas, comprising: said pressurized vessel; a heatedmelting pot inside said pressurized vessel for melting a siliconmaterial into the silicon melt; a capillary tube providing a continuouspath from said heated melting pot for the silicon melt; a set ofsonotrodes for producing the ultrasonic field underneath a bottom ofsaid capillary tube, wherein said capillary tube feeds the silicon meltinto said ultrasonic field, and said capillary tube has a maximumdistance of 50 mm from one field node of said sonic field; a heat shieldadjacent said bottom to insulate said capillary tube from saidultrasonic field; and all surfaces in said pressurized vessel coming incontact with the globular grains comprise at least one of anon-contaminating material, a coating with a liner made of anon-contaminating material and a coating with a non-contaminatingmaterial.
 19. A device according to claim 18, wherein said ultrasonicfield is generated by at least two sonotrodes being arranged at aminimum distance of nine field nodes between each other.
 20. A deviceaccording to claim 18, wherein said capillary tube is provided withheating means.
 21. A device according to claim 18, wherein the distancefrom the walls of said pressurized vessel to an atomisation zone foratomizing said silicon melt is chosen so that the globular grains arecompletely solidified before the globular grains hit said walls.
 22. Adevice according to claim 18, wherein at least one of graphite, siliconcarbide and quartz glass are used as said non-contaminating material.23. A device according to claim 18, wherein the silicon is fed into saidheated melting pot via a solid matter valve.
 24. A device according toclaim 18, wherein the bottom of said pressurized vessel is provided witha discharge means for discharging the high-purity silicon.
 25. A deviceaccording to claim 24, wherein a cellular wheel sluice is provided assaid discharge means.
 26. A device for producing globular grains ofhigh-purity silicon by atomizing a silicon melt in an ultrasonic fieldinside a pressurized vessel filled with inert gas, the devicecomprising: said vessel; a heated melting pot inside said vessel formelting a silicon material; a set sonotrodes underneath said heatedmelting pot for producing a sonic field; a capillary tube between saidheated melting pot and said sonic field, wherein said capillary tube isprovided with a heating means, and the silicon melt is fed into theultrasonic field through said capillary tube; a heat shieldheat-insulating said capillary tube from said ultrasonic field; and allsurfaces coming in contact with the globular grains in said pressurizedvessel comprising a non-contaminating material.
 27. A device accordingto claim 26, wherein said ultrasonic field is generated by at least twosonotrodes being arranged at a minimum distance of nine field nodesbetween each other.
 28. A device according to claim 26, wherein saidcapillary tube has a maximum distance of 50 mm from one yield node ofsaid ultrasonic field.
 29. A device according to claim 26, wherein thedistance from the walls of said pressurized vessel to an atomisationzone for atomizing said silicon melt is chosen so that the globulargrains are completely solidified before the globular grains hit saidwalls.
 30. A device according to claim 26, wherein at least one ofgraphite, silicon carbide and quartz glass are used as saidnon-contaminating material.
 31. A device according to claim 26, whereinthe silicon is fed into said heated melting pot via a solid mattervalve.
 32. A device according to claim 26, wherein the bottom of saidpressurized vessel is provided with a discharge means for dischargingthe high-purity silicon.
 33. A device according to claim 32, wherein acellular wheel sluice is provided as said discharge means.